There are 28 families (extant and fossil) of scale insects representing more than 7,300 species worldwide. They belong to the group of insects known as the Sternorrhyncha which include related insects such as aphids, psyllids, and whiteflies. The term “Sternorrhyncha” (from the Greek sternon meaning chest or breast and rhynchos meaning nose, snout, or muzzle) refers to the location of the mouthparts. The mouthparts are found on the ventral side of the insect between the fore-coxae and project backwards.

    Scale insects are among the most unusual insects known. Adult female scales appear quite unlike a “typical” insect. They are sacklike with no well-defined head, thorax, or abdomen. They are neotenic, that is they resemble the immature stages as an adult. Adult females are wingless and may or may not have legs. Female scales usually have two or three immature instars. The female lays eggs or first instars either in a cavity under her body or in a waxy covering that may of may not be attached to her body. First instars are mobile and serve as principle agents for dispersion and seeking out suitable feeding sites; other immature instars generally are sessile (with some notable exceptions).

    Adult male scales appear more insectlike with a better defined head, thorax and abdomen. Although some adult males are wingless, they usually have a pair of forewings and usually have a pair of very small hind wings. Male scales usually have four immature instars. Male scales, though part of the Hemiptera which have incomplete metamorphosis, have their own specially derived form of a complete metamorphosis with one or two pupa-like instars. These pupa-like instars (called the prepupa and pupa) develop in a waxy enclosure or test that is produced initially by the second instar. The pupa-like instars and the adult males do not feed and have only rudimentary mouthparts. Adult females may live for an extended period and usually have well-developed mouthparts. Conversely, adult males are short lived and lack mouthparts.

    Most scale insects produce a waxy secretion that covers the body either as a protective structure (called a scale cover or test) detached from the body or as a secretion that covers the body surface. This waxy secretion may vary from a thin translucent sheet to a thick, wet mass or a powdery, bloom-like secretion.

    Scales have the greatest diversity of sperm structure and sex determining chromosome systems of any known group of organisms. Reproductive systems include hermaphroditism, seven kinds of parthenogenesis, and six major types of sexual chromosome systems. One group even has a placenta-like structure in the female that is used to feed first-instar males.

    Scale insects are serious plant pests and as small, highly cryptic components of the plant ecosystem, frequently go undetected until they have caused substantial damage. Scale insects are particularly difficult to detect in quarantine inspections especially at low population levels. They are very important as agricultural pest of perennial plants and can cause serious damage to nut and fruit trees, woody ornamentals, forest vegetation, greenhouse plants, and house plants. Plant damage is usually caused by removal of plant sap. However, scale insects can also transmit plant pathogens and toxins that may further reduce plant vigor and eventually kill the plant. In addition, scale insects produce large quantities of a sticky substance that can cover the leaves. This sticky substance, which is called honeydew, provides an excellent substrate for the growth of sooty mold fungi. Sooty mold fungi in turn can reduce photosynthesis and plant vigor.

    Scales also can be beneficial. They have been used as sources of dyes (cochineal scales, gall-like scales, giant scale, and lac scale), of shellac and lacquer-like substances (lac scales and giant scales), of candle wax (soft scales), of the manna of the Israelites (mealybugs), of pearls for necklaces (ground pearls or giant scales), and even chewing gum (ornate pit scale). Cochineal scales and mealybugs have been used in the control of noxious weeds.